Illtyd (Welsh Illtud Farchog, 'Illtud the Knight'; Latin Iltutus) was — per the 12th-century Vita Sancti Iltuti and the substantial earlier Welsh hagiographic tradition — a Brittonic-Celtic Christian of the late 5th century. The tradition holds that he was originally a soldier in the post-Roman Brittonic military, the cousin of King Arthur (in the Welsh-Latin Arthurian tradition), and was converted to monastic life after a near-death hunting accident around 480 AD.
He travelled to Llanilltud Fawr (Latinized Llantwit Major) in the Vale of Glamorgan in southern Wales, where he established around 490 AD the great monastic-school of Llanilltud Fawr — the Welsh monastic school and one of the most influential educational institutions of the entire post-Roman Brittonic-Christian world. Across the late 5th and 6th centuries the school of Llanilltud Fawr produced an extraordinary succession of Welsh and Breton saints — St. Samson of Dol, St. Paul Aurelian of Brittany, St. Gildas the Wise, St. David of Wales (see David's entry already in the catalogue), and many others.
Illtyd is the father of the Welsh Catholic monastic-school tradition. He died around 505 AD at Llanilltud Fawr. The medieval Parish Church of St. Illtud at Llantwit Major (the continuous Welsh Christian community) preserves substantial early-Christian inscribed stones from his original monastic complex — including the famous Houelt Cross, one of the early-medieval Welsh Catholic-Christian inscribed monuments.